Add multi-input-multi-output converter

[henhuy]

This PR adds a new converter which can group inputs and outputs into multiple groups.
Inputs/outputs within a group can be exchangeable used to provide group flow.

Compare conversion_plus technology from calliope: https://calliope.readthedocs.io/en/v0.6.8/user/advanced_constraints.html#the-conversion-plus-tech

For each input/output group a new pyomo Variable/Index has to be set up.

[pep8speaks]

Hello @henhuy! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2023-11-22 10:21:58 UTC

[p-snft]

I don't get why this is needed. Everything sketched at https://calliope.readthedocs.io/en/v0.6.8/user/advanced_constraints.html#the-conversion-plus-tech could be modelled using solph Converters (sometimes additional Buses might be needed). I would rather suggest to add a workflow to have sub-graphs grouping different nodes (and allow hiding the respective internal Flows).

[henhuy]

Hey @p-snft - I see what you are suggesting and like the idea of a more general approach. Started an issue for that: #1015
Maybe you could have a look, if I understood you correctly and my implementation idea is a good way to go.

[p-snft]

I think this is equivalent to multiplying the conversion_factors of the converters that are put in series and is actually not needed. Maybe, a tutorial, a facade or a helper function would be an option?